This invention relates to fuel control systems and more particularly to fuel control systems for gas turbine engines of the type providing vehicle propulsion.
Fuel control systems for modern gas turbine engines of the type used in vehicle propulsion involve complex computation of a variety of engine parameters to provide proper metering of the fuel to the engine combustor. Such systems are additionally complex for engines having multiple spools as opposed to single spool engines. The parameters which must be sensed and operated upon to provide proper engine fuel flow are in part; spool speeds, engine inlet temperature, ambient pressure, turbine inlet temperature, compressor discharge pressure and surge bleed valve position. The problem is further complicated in the case of small engines, principally used in general aviation aircraft where the crew consists of only a pilot and co-pilot and no flight engineer is available to control and monitor engine performance. In this case, it is required that engine control and monitoring be as simple and automatic as possible so as not to require an excessive amount of the crew's time. One of the principal problems to be avoided in the operation of such engines is the condition of compressor surge in which a discontinuity of fluid flow through the compressor causes violent fluid pressure fluctuations which can cause damage to the engine and eventually total destruction.
Heretofore, fuel control systems for gas turbine engines have been designed to be conservative in order to ensure protection against surge. That is, total output power would be sacrificed in order to ensure safe engine operation. The present invention overcomes these disadvantages by providing a fuel control system for a multiple spool gas turbine engine in which the power output is controlled by a single lever in which the power output is a linear function of lever position. Engine parameter monitoring scheduling and computation of optimum fuel flow are provided by an automated electronic system. In particular, the surge valve control is arranged to provide maximum thrust at each power lever setting by means of a closed loop control involving fluid flow control logic and the surge valve position signal. The operator need only position the lever to indicate the percent of maximum thrust desired and the system will provide maximum safe acceleration to that power level and maintain the level until the power level is adjusted to a new setting.